Too late to add this idea to any existing guns but for future guns I had an idea that could potentially balance out stronger guns.
That idea is to add a percentage to bullet damage. For example, say a gun did 10 damage default, there would be a 20-35% chance for the bullet to do 9 damage, so it is not a guaranteed 5 shot kill, but it still has the potential to be.

This would make those stronger guns more balanced as whilst its effectiveness would still rely on skill, it would give the opponent a better chance as the bullet might not always do the highest amount of damage so they could end up surviving one hit longer.

But yea that's pretty much it, if this was ever to be implemented it would only be for higher damaged guns, for low damage guns 1 damage less or more can make a pretty big difference so probably wouldn't be the best to have that.